How to place and merge meshes upon another one (and then delete them)
You could do it with some cuttings, duplicate and snaps, but maybe the simplest ways is to use the Knife Project tool:


Separate your 3 faces with P (Separate) > Selection. Now your 3 faces are a separate object.
Select this object, then the main object, and go in Edit mode
Switch to Ortho Front View.
In the Tools panel, click on the Knife Project button. You've created 3 faces. You can delete the faces in order to create doors.
You may want to use the knife with Z (cut through) and C (cut orthogonally) options activated to create additional edge loops.